1966 c4 is not shifting well from second to third - Ford Mustang Forum
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post #1 of 7 (permalink) Old 08-15-2019 Thread Starter
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1966 c4 is not shifting well from second to third

Hello,
Suddenly my green dot trans is having difficulty shifting from second to third. The engine revs up and will shift if I let up on the throttle or if I let it rev. I had a transmission shop repair a shift shaft leak 4k miles ago and it was working fine until now. I checked the fluid level which was high and there were some bubbles on the dipstick which made me think it was cavitating. I drained 8oz of atf out, which put it at the proper level. I replaced the modulation valve and made sure the vacuum hose was in good shape (even put a zip tie clamp on the hose). Now I’ve drained 16oz and added “Bar’s leaks” additive that says it will improve shifting. Nothing has helped. Should I pull the transmission and rebuild it or could it be the torque converter?
I appreciate any and all ideas.

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post #2 of 7 (permalink) Old 08-15-2019
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Originally Posted by Tlkmap View Post
Hello,
Suddenly my green dot <acronym title="transmission">trans</acronym> is having difficulty shifting from second to third. The engine revs up and will shift if I let up on the throttle or if I let it rev. I had a transmission shop repair a shift shaft leak 4k miles ago and it was working fine until now. I checked the fluid level which was high and there were some bubbles on the dipstick which made me think it was cavitating. I drained 8oz of atf out, which put it at the proper level. I replaced the modulation valve and made sure the vacuum hose was in good shape (even put a zip tie clamp on the hose). Now I’ve drained 16oz and added “Bar’s leaks” additive that says it will improve shifting. Nothing has helped. Should I pull the transmission and rebuild it or could it be the torque converter?
I appreciate any and all ideas.
Is the complaint specifically that the engine speeds up during shifting to 3rd.? This indicates slippage, most likely in the High Clutch. No Converter trouble.

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post #3 of 7 (permalink) Old 08-16-2019 Thread Starter
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Yes, exactly. Since the transmission has never been apart as far as I know, should I do a complete rebuild? Sounds like I need to remove and rebuild it? Is there a specific rebuild kit? Is $400 too much to pay? There is a local guy on craigslist that advertises $400 to rebuild. Does not include removal.
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post #4 of 7 (permalink) Old 08-16-2019
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The high gear clutches have worn over 75% of the lining off ( at least) . A transmission that old should come completely apart and have all of the internal rubber O ring seals replaced during the full rebuild. It would be a great time to add an inexpensive shift kit too. The situation will only get worse from where you are at now.
Randy
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post #5 of 7 (permalink) Old 08-16-2019 Thread Starter
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Ok. Thanks for the comeback!
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post #6 of 7 (permalink) Old 08-17-2019
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When you overfill, and there's a lot of foam, any transmission will have trouble shifting. The fluid becomes 'springy' because of the air in it, and when the hydraulic actuators try to clamp down on the bands, etc., they can't deliver full pressure. If your transmission has been rebuilt in recent history, the friction materials don't care about whether you use Type F fluid too much, but that specification has a lot of anti-foam properties which is nice too.

If it calms down, and drives normally now that you got the fluid levels right, then good - but if the friction material's gotten damaged due to excessive slip, then it looks like a good time to do one. Thankfully, the C4 is pretty simple inside compared to most transmissions.

A shift kit will allow for firmer shifts, and that helps the friction materials last longer. (If that sounds counterintuitive, just think about what happens to a clutch when you slip it too much. Same thing.)

Anyway, let us know how it all turns out!

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post #7 of 7 (permalink) Old 08-17-2019 Thread Starter
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Thanks! Wilco
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